For those with a long layover at Chicago O'hare airport, wanting to explore the city, you might be surprised to find out the airport does not offer any luggage storage services! So keep your carry-on luggage light if possible. If you have access to one of the premium airline lounges, you can store your luggage there while exploring the city for the day. There is a Hilton hotel at Chicago O'hare, with some travelers reportedly leaving their bags there, just ask at the front desk (HHonors loyalty membership would help). For those with train connections, Union Station downtown offers storage lockers with a maximum cost of $12 for large size or $16 for jumbo sized lockers up to 24hrs. Check the locker sizes below to see if your luggage fits as 'jumbo' can be interpreted in many ways. Amtrak also has a premium lounge and will store your bags there for free if you have access.

Price:$4 for the first hour$12 for 3 hours or more (24 hours max)JUMBO LOCKERSize:35.25" high x 18" wide x 32.28" deep

Price:$6 for the first hour$16 for 3 hours or more (24 hours max)

If none of these options work, then your best bet is to head to one of the museums that offer coat check services and offload your bags there to explore the city. The Art Institute of Chicago near Grant Parkis a good one and has a large coat check/baggage storage area costing just $1 per item. This of course means you are lugging your stuff to and from the museum though...